Ive lived in hot countries, and I swear long journeys with cats are better done either in the evening the early morning or at night time, but I understand that isnt always possible. We were at a cat show in Germany this past Saturday, and it was 29 degrees (in the shade) it was hideous for the cats.
Theres something I would definitely recommend you take with you, but Im not sure what theyre called. You know those blue plastic ice-pack things that you put in the freezer then put them into cool boxes to keep your picnic cool??? Well have as many of them on hand as possible. Then when they are a bit defrosted, but still cool, wrap them in a tea-towel and position one or two in the cats travelling crate. But do make sure he/she has enough space to that he doesnt have to lie on the frozen elements, but can if they choose to. Even if they are stood up against the sides of the crate theyll help keep the cat cool.
Id also make sure you have something thin to cover the travelling crate in if its direct sunlight. Something breathable like a thin sheet of muslin is perfect. You can buy special spill-proof cat bowls to keep some water in the crate. Before these travel bowls were invented I used to just pop a soaking wet face cloth in a cat bowl
if theyre really thirsty they will lick from it, and you get no mess or spillage. Dont use any fleece or wool blankets in the travel crate, just something cotton or linen or some natural fiber.
If the cat appears to be overheating.. well keep stress to a minimum as that will make it worse, and try and stop somewhere (I did it once at a public toilet on a motorway) got my hubby to guard the door while I let the cat out of the crate to cool down on the tiled floor. Also slightly wetting (dampening but not soaking) the cats fur with a damp face cloth will help. Pay special attention to the pads of their feet and their underarms, getting those spots cool, cools the cats core temperature. Though nothing drastic like ice water, just cold water on a face cloth. At a pinch
nip into a supermarket and get frozen peas or any bag of frozen veg
wrap it in a tea-towel and position that in the cats carrier (again, make sure the carrier is big enough that he can get away from the ice-pack if its too cold).
You can also buy battery operated fans. When we used to live in the desert I would use one of these clipped on to the outside of the travel crate with an ice-pack thing in front of it on the inside of the crate. That way the breeze blown off the fan, cools down as it passes over the ice pack. It really reduces the temp inside the travel crate. Even with the aircon on inside a car, the travel crate heats up as the aircon can't cool the inside of the crate and the heat the cat generates can't escape.
oh geez, this has turned out very long and very anal

, lol. Ive just had so many near disasters with cats and travel in heat that I am ultra cautious now.
I'll keep my fingers crossed for you that Saturday is dull and cloudy but dry!!